Surgical apparatus for implantation of a partial or total knee prosthesis

The present invention relates to a surgical apparatus for implantation of a partial or total knee prosthesis.

A total knee prosthesis conventionally comprises: <ul><li id="ul0001-0001" num="0000"><ul><li id="ul0002-0001" num="0003">a tibial part generally composed of a metal platform to be firmly connected to the resected upper end of the tibia, and of a polyethylene runner which rests, in a fixed or movable manner, on the upper face of the platform,</li><li id="ul0002-0002" num="0004">a femoral component in the form of a generally metal hood which, on one side, is firmly connected to the resected lower end of the femur and which, on the other side, delimits two convex surfaces that approximately reproduce the geometry of two internal and external anatomical femoral condyles and are designed to rest in an articulated manner on two corresponding concave surfaces formed by the tibial runner, and</li><li id="ul0002-0003" num="0005">a patellar implant firmly connected to the anatomical patella, on its posterior face, that is to say on its face directed towards the femur, after resurfacing of said face.</li></ul></li></ul>

The success of the implantation of such a prosthesis and its subsequent mechanical hold during its working life are very much dependent on the relative positioning of the implanted prosthetic elements. In practice, it is found that the dislocations of the patella fitted with the prosthesis are among the most common causes of failure. The patellar implants are in fact conventionally fitted in place without any great consideration being paid to the femoral and tibial elements of the prosthesis, the surgeon generally being content to centre the patellar implant roughly on the resected posterior face of the anatomical patella. It is true that this bone is awkward to manoeuvre since it is held by the tendon of the quadriceps and the latter has to be turned aside in order to permit resurfacing of the then dislocated patella.

US-A-2003/0212403 proposes a method of fitting a total knee prosthesis intended to limit the invasive nature of the surgery. To this end, this document proposes spatially marking out simultaneously the bones of the tibia, femur and patella and using these markers to guide the surgeon's manoeuvres, without his having to completely incise the knee joint of the patient. The trauma suffered by the patient is thus reduced and the work of the surgeon is made easier. However, the positioning of the patellar implant is decided a priori by the surgeon solely on the basis of the anatomical information relating to the patella on which the prosthesis is to be fitted, without taking into consideration the implantation positions of the tibial and femoral prosthetic components. The risks of dislocation of the patella thus fitted with the prosthesis are therefore similar to those of a prosthesis implanted by a more invasive conventional method.

WO-A-02/36031 proposes a system for determinating the position of a knee prosthesis to be implanted, this system being able both to specially mark out the bones of the knee and to simultaneously display the patella to be fitted with the prosthesis and the lower end of the femur after that this end is fitted with the femoral prosthesis. This reference does not provide with any possibility of virtual representation of implants, contrary to reference WO-A-2004/001569 which, however, does not concern an improvement of the fitting of a pattelar implant. In these two references, the proposed representations are only in elevations of the bones, especially either in an elevation corresponding to the internal face of the patella or in an axial or frontal elevation of the intercondylar area of the femur in WO-A-02/36031. Such representations do not provide a sufficient precision to efficiently position the implants of a knee prosthesis, especially to limit the risks of dislocation of the patella thus fitted with the prosthesis.

The object of the present invention is to make available a surgical apparatus which affords simple, rapid and low-cost assistance to the surgeon during implantation of a partial or total knee prosthesis with a view to limiting the risk of subsequent dislocation of the patellar implant, by means of better adaptation to the other components of the implanted prosthesis, in particular the femoral implant.

To this end, the invention relates to a surgical apparatus for implantation of a partial or total knee prosthesis comprising at least a patellar implant and a femoral implant, said apparatus having: <ul><li id="ul0003-0001" num="0000"><ul><li id="ul0004-0001" num="0011">means for peroperative spatial marking-out of the bones of the patella and femur to be fitted with the prosthesis,</li><li id="ul0004-0002" num="0012">means for peroperative representation that are designed to show, in a transverse plane passing both through the patella and through an intercondylar trochlea of the femoral implant, a simultaneous and relative representation of the patella which is to be fitted with the prosthesis and is provided virtually with the patellar implant, and of the lower part of the femur which is to be fitted with the prosthesis and is provided virtually with the femoral implant.</li></ul></li></ul>

Using the apparatus according to the invention, the surgeon, during the surgical intervention, is able to view the representation of the patellar implant implanted virtually on the patella of a patient relative to the representation of the femoral implant implanted virtually on the femur of the patient and can thus check the congruence between the patellar implant to be implanted and the intercondylar trochlea of the femoral implant to be implanted. This is because the surgeon seeks to obtain the best possible congruence for the purpose of limiting the risk of any subsequent luxation of the patella with respect to the femoral implant, both on the internal side and the external side of the trochlea. This check is particularly effective in the transverse plane, that is to say substantially perpendicular to the tendon of the quadriceps and to the lateral tendons of the knee joint, which passes both through the bone of the patella and through the intercondylar trochlea of the femoral implant, since the external and internal transition zones between the trochlea and the prosthetic condyles are then clearly visible. In other words, the fact that this transverse plane passes through the bone tissue constituting the patella and the intercondylar trochlea provides a notable precision for the representation of the bones and of their virtual implants, which enables to quickly and efficiently figure the centering of the patellar implant with respect to the intercondylar trochlea, and the appropriate controls in order to implant this patellar implant on the patella. In the event that the representation means of the surgical apparatus according to the invention indicate to the surgeon that the expected congruence is insufficient, the surgeon can decide on surgical manoeuvres aimed at improving the implantation configuration, especially by modifying the geometry of implantation of the patellar implant and, if appropriate, that of the femoral implant.

According to other characteristics of this apparatus, taken either singly or in any of the technically possible combinations: <ul><li id="ul0005-0001" num="0000"><ul><li id="ul0006-0001" num="0015">the transverse plane in which the patella and the femur are represented by the representation means corresponds to a substantially median plane of the patella;</li><li id="ul0006-0002" num="0016">it includes a memory containing data relating to a preferential predetermined positioning of the patellar implant relative to the femoral implant for all the anatomically allowable configurations of flexion-extension of the knee, and wherein the representation means are designed to represent, in the transverse plane and in a simultaneous and relative manner, both the patella fitted virtually with the prosthesis, the femur fitted virtually with the prosthesis, and an image for assisting the surgeon, represented over at least one part of the patellar implant, this assistance image being positioned with respect to the virtual femoral implant, depending on the knee's flexion-extension configuration considered, as a function of the data on preferential positioning that are contained in the memory;</li><li id="ul0006-0003" num="0017">the representation means are designed to represent the patella fitted virtually with the prosthesis, the femur fitted virtually with the prosthesis and, if appropriate, the image for helping the surgeon obtain different configurations of flexion-extension of the knee, the transverse plane in which these elements are represented by these means being fixed relative to the patella;</li><li id="ul0006-0004" num="0018">the representation means are designed to represent said elements over a continuous course of flexion-extension of the knee, in particular over the course of maximum flexion-extension that is anatomically allowable;</li><li id="ul0006-0005" num="0019">it comprises means of adjustment of the virtual implantation of the patellar implant on the patella;</li><li id="ul0006-0006" num="0020">in the case where the patellar implant is to be fitted level with a posterior cutting of the patella, the adjustment means are designed to modify the distance between a predetermined point of the anterior face of the patella and the patellar sectioning line in the transverse plane, the angle value of the patellar sectioning line relative to a predetermined direction, and/or the position of the patellar implant along the patellar sectioning line;</li><li id="ul0006-0007" num="0021">in the case where a set of several patellar implants of different respective sizes is provided, the adjustment means are designed to change the size of the implant shown;</li><li id="ul0006-0008" num="0022">it comprises: <ul><li id="ul0007-0001" num="0023">means for selecting and memorizing at least one configuration of implantation of the patellar implant set by the adjustment means,</li><li id="ul0007-0002" num="0024">an ancillary used for resurfacing the anterior face of the patella and equipped with spatial marking means, and</li><li id="ul0007-0003" num="0025">means for comparing the resurfacing action effected by this ancillary with the data from the selecting and memorizing means;</li><li id="ul0007-0004" num="0026">it additionally comprises other means for simultaneous and relative representation of the patella which is to be fitted with the prosthesis and is provided virtually with the patellar implant, and of the lower part of the femur which is to be fitted with the prosthesis and is provided virtually with the femoral implant, in a sagittal plane passing both through the patella and the trochlea of the femoral implant.</li></ul></li></ul></li></ul>

The invention also relates to a surgical method for implantation of a partial or total knee prosthesis comprising at least a patellar implant and a femoral implant, in which method the following steps are carried out in succession in the peroperative period: <ul><li id="ul0008-0001" num="0000"><ul><li id="ul0009-0001" num="0028">the bones of the patella and femur of a patient to be treated are spatially marked out,</li><li id="ul0009-0002" num="0029">the patella which is to be fitted with the prosthesis and is provided virtually with the patellar implant and the lower part of the femur which is to be fitted with the prosthesis and is provided virtually with the femoral implant are represented in a simultaneous and relative representation, this representation being effected in a transverse plane passing both through the patella and also through an intercondylar trochlea of the femoral implant,</li><li id="ul0009-0003" num="0030">the virtual implantation of the patellar implant on the patella and, if appropriate, the virtual implantation of the femoral implant on the femur is adjusted such that the posterior part of the patellar implant is received congruently in the femoral intercondylar trochlea,</li><li id="ul0009-0004" num="0031">the configuration of implantation of the patellar implant obtained according to the adjustment is selected and memorized, and</li><li id="ul0009-0005" num="0032">the real patellar implant is implanted on the patella of the patient according to the configuration of implantation that has been selected and memorized.</li></ul></li></ul>

According to an advantageous characteristic of this method: <ul><li id="ul0010-0001" num="0000"><ul><li id="ul0011-0001" num="0034">in relation to the position of the femoral implant virtually implanted on the spatially marked-out femur, the position of the patellar implant virtually implanted on the spatially marked-out patella is compared with a predetermined position of this patellar implant in which position the patellar and femoral implants are positioned in a congruent manner, and</li><li id="ul0011-0002" num="0035">the virtual implantation of the patellar implant on the patella is adjusted in such a way that its position coincides substantially with the predetermined position.</li></ul></li></ul>

The invention will be better understood from reading the following description which is given solely by way of example and in which reference is made to the attached drawings, in which:

<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic view of part of a surgical apparatus according to the invention, applied to the knee of a patient to be operated on;

<figref idrefs="DRAWINGS">FIG. 2</figref> is a perspective view of a total knee prosthesis to be implanted by means of the apparatus from <figref idrefs="DRAWINGS">FIG. 1</figref>;

<figref idrefs="DRAWINGS">FIG. 3</figref> is a view, according to the arrow III indicated in <figref idrefs="DRAWINGS">FIG. 2</figref>, of part of the knee prosthesis in a possible state of functioning;

<figref idrefs="DRAWINGS">FIG. 4</figref> is a schematic side view of the patient's knee joint;

<figref idrefs="DRAWINGS">FIG. 5</figref> is a section along a plane P in <figref idrefs="DRAWINGS">FIG. 4</figref>;

<figref idrefs="DRAWINGS">FIG. 6</figref> is a view illustrating the representation provided by the apparatus from <figref idrefs="DRAWINGS">FIG. 1</figref>, in the plane P;

<figref idrefs="DRAWINGS">FIG. 7</figref> is a sagittal cross section, along a plane S in <figref idrefs="DRAWINGS">FIG. 2</figref>, of the femoral component of the prosthesis from <figref idrefs="DRAWINGS">FIG. 2</figref>;

<figref idrefs="DRAWINGS">FIG. 8</figref> is a diagram showing the trajectory of the patellar implant of the prosthesis from <figref idrefs="DRAWINGS">FIG. 2</figref> relative to the femoral implant, during a flexion-extension movement of the knee prosthesis; and

<figref idrefs="DRAWINGS">FIG. 9</figref> is a view illustrating another representation provided by the apparatus from <figref idrefs="DRAWINGS">FIG. 1</figref>, in the plane S.

The surgical apparatus <b>1</b> in <figref idrefs="DRAWINGS">FIG. 1</figref> comprises a computer <b>2</b> connected to a unit for transmission and reception of infrared radiation. This unit comprises a sensor <b>3</b> connected to the computer, and an infrared source <b>4</b> covering the operating field in which a part of a patient's knee that is to be treated is shown. The knee comprises the lower part of a femur F, the upper part of a tibia T, the tendon of the quadriceps Q which carries the bone of the patella R, and the lateral tendons L which connect the femur and the tibia.

The allow the computer <b>2</b> to spatially mark out the bones of the femur F, tibia T and patella R, the apparatus <b>1</b> comprises respective groups of markers <b>5</b>, <b>6</b> and <b>7</b> which return, passively, the infrared radiation in the direction of the sensor <b>3</b>. Each group of markers <b>5</b>, <b>6</b> or <b>7</b> forms a three-dimensional marking system allowing the assembly made up of computer <b>2</b> and sensor <b>3</b> to spatially track the respective displacements of the femur, tibia and patella. The use of such markers is well known in the field of orthopaedics, for example in document EP-A-1 249 213, and they will not be described in any more detail below.

Each group of markers <b>5</b>, <b>6</b> or <b>7</b> is fixed to the bone of the femur, the tibia or the patella by means of one or more rigid pins. These pins are positioned on the bones in such a way as to leave their markers permanently visible to the sensor <b>3</b>.

The computer <b>2</b> of the apparatus <b>1</b> is also connected to one or more screens <b>8</b> for displaying information useful to the surgeon, in particular information relating to the relative position of the bones F, T and R, and other data described further below, preferably in the form of graphic representations, as detailed hereinafter.

The apparatus <b>1</b> also comprises control means <b>9</b>, for example in the form of a pedal that can be activated by the surgeon's foot.

The surgical apparatus <b>1</b> additionally comprises other components which will be detailed hereinafter in the description of a detailed example of the use of this apparatus for implantation of a total knee prosthesis <b>10</b> represented alone in <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref>. This prosthesis is made up of a tibial part <b>11</b> to be implanted in the bone of the tibia T, a femoral component <b>12</b> to be implanted in the bone of the femur F, and a patellar component <b>13</b> to be implanted on the bone of the patella R.

The tibial part <b>11</b> of the prosthesis <b>10</b> comprises a rigid and generally flat platform <b>111</b> provided, on one side, with a stub <b>112</b> for anchoring it in a previously resected upper end of the tibia T and equipped, on the opposite side, with a plateau or roller <b>113</b> which is connected to the platform in a fixed or movable manner depending on the type of prosthesis <b>10</b>. On its face intended to be directed towards the femoral component <b>12</b>, the plateau <b>113</b> defines two concave surfaces <b>113</b>₁and <b>113</b>₂which are designed to receive in an articulated manner two condyles, namely the internal condyle <b>121</b> and external condyle <b>122</b>, which are formed by the femoral implant <b>12</b> and reproduce approximately the geometry of two anatomical femoral condyles. The condyles <b>121</b> and <b>122</b> delimit between them a femoral trochlea <b>123</b> which is intended to receive the patellar implant <b>13</b>, as is shown in <figref idrefs="DRAWINGS">FIG. 3</figref>. For this purpose, the patellar implant comprises a button <b>131</b> having the overall shape of a spherical cap, and three stubs <b>132</b> which are used for anchoring in the posterior part of the patella and project forwards from the substantially plane anterior face of the button <b>131</b>, whose centre is indicated by O. The concave curvature of the femoral trochlea <b>132</b> and the convex curvature of the patellar button <b>131</b> are dimensioned in such a way that the patellar implant <b>13</b> bears congruently on the femoral implant <b>12</b>. This congruent contact thus permits, between the implants <b>12</b> and <b>13</b>, a relative movement of translation along the femoral trochlea <b>123</b>, as is shown by the arrows F₁and F₂in <figref idrefs="DRAWINGS">FIG. 2</figref>, and also movements of relative pivoting about the condyles <b>121</b> and <b>122</b>, as is indicated by the double arrow F₃in <figref idrefs="DRAWINGS">FIG. 3</figref>.

The prosthesis <b>10</b> described above is given only by way of example, and other prostheses of a different geometry and/or of a different nature can be implanted by means of the apparatus <b>1</b> according to the surgical implantation method described below. In particular, the invention applies to the implantation of partial knee prostheses, in particular to femoro-patellar prostheses composed of a patellar implant and of a femoral implant designed to bear directly on the upper end of the anatomical tibia and currently called a “trochlear implant”.

In a first step, the surgeon makes an incision in the patient and collects a certain quantity of data relating to the anatomical geometry of the femur F, tibia T and patella R of the patient. For this purpose, different data acquisition means can be employed. By way of example, the surgeon uses a tracer <b>20</b> which is marked by the assembly of computer <b>2</b> and sensor <b>3</b> and which is previously calibrated. This tracer is moved to noteworthy sites of the bones to be marked out and, at each of these locations, the surgeon activates the control pedal <b>9</b> in such a way that the computer <b>2</b> records the position of the tracer and, in this way, deduces the anatomical characteristics of the femur F, tibia T and patella R. Based on these data, the tracking of the markers <b>5</b>, <b>6</b> and <b>7</b> and pre-recorded data relating to a base geometry of the knee, the computer <b>2</b> is able to spatially mark out the bones F, T and R and monitor their relative displacements.

During this step of data acquisition, the tendon of the quadriceps Q is turned aside, as is shown in <figref idrefs="DRAWINGS">FIG. 1</figref>, in such a way that the reflecting markers <b>7</b> are visible to the sensor <b>3</b>.

In a second step, the surgeon indicates to the computer <b>2</b> the model of the prosthesis <b>10</b> that he intends to implant. The choice of this prosthesis is either left to the entire discretion of the surgeon or is suggested by the computer, taking into consideration the morphological features, in particular the bone features, of the patient being operated on. The remainder of the description is based on the hypothesis that the surgeon selects the prosthesis <b>10</b> from <figref idrefs="DRAWINGS">FIGS. 2 and 3</figref> as the one that he wishes to implant.

In a third step, the computer <b>2</b> supplies to the surgeon, via the screen <b>8</b>, a virtual representation of the implantation configurations of the femoral component <b>12</b> and patellar component <b>13</b> of the prosthesis <b>10</b>. More precisely, the computer displays on its screen <b>8</b> the patient's patella R which is provided virtually with the patellar implant <b>13</b>, and the lower part of the femur F which is provided virtually with the femoral implant <b>12</b>. To allow the surgeon to assess the congruency between the prosthetic button <b>131</b> and the prosthetic femoral trochlea <b>123</b>, this representation is effected in a transverse plane P passing through both the patella R and also the prosthetic femoral trochlea <b>123</b>.

To fully understand the benefit of this representation, <figref idrefs="DRAWINGS">FIG. 4</figref> shows the anatomical knee joint from <figref idrefs="DRAWINGS">FIG. 1</figref>, it being understood that the joint shown does not display any noticeable structural anomaly. The joint is shown in the configuration of extension, so that the patella R bears on the anterior face of the lower end of the femur F at the level of the anatomical femoral trochlea F₁₂₃. By graphically sectioning this joint along the plane P indicated in <figref idrefs="DRAWINGS">FIG. 4</figref>, one observes in <figref idrefs="DRAWINGS">FIG. 5</figref> the anatomical congruence of the posterior face of the patella R in relation to the anatomical trochlea F₁₂₃. This plane P is transverse to the patella, that is to say it is substantially perpendicular to the tendon of the quadriceps Q and to the lateral tendons L of the knee joint, thus cutting through the patella at the area of its internal and external flanks. By means of this representation in the plane P of both the patella R provided virtually with the patellar implant <b>13</b> and also of the femur F provided virtually with the femoral implant <b>12</b>, the surgeon is provided with graph information representing the more or less congruent implantation of the femoral and patellar components of the prosthesis <b>10</b>. An example of this representation is given in <figref idrefs="DRAWINGS">FIG. 6</figref>.

In practice, the transverse plane P corresponds advantageously to a substantially median plane of the patella R, that is to say a transverse plane on either side of which the bone material constituting the bone of the patella R is distributed in an almost identical manner. In this case, the plane P corresponds substantially to a plane of symmetry of the patellar button <b>131</b>.

Thus, during the third operating step, the surgeon checks whether the virtual implantations of the patellar component <b>13</b> and femoral component <b>12</b> place the patellar button <b>131</b> congruently in the prosthetic femoral trochlea <b>123</b>. If, as in <figref idrefs="DRAWINGS">FIG. 6</figref>, this congruent configuration is not obtained, the surgeon concludes that the implantation of the patellar implant <b>13</b> on the patella R and/or the implantation of the femoral implant <b>12</b> on the femur F are unsatisfactory and he corrects them. For this purpose, the computer <b>2</b> is equipped with means for adjustment of these virtual implantations, these means being controlled for example by touching the screen <b>8</b>. In particular, the surgeon is then able to adjust the position of the line R_c1sectioning the patella in the plane P, in particular its distance e_pfrom the summit of the patella, and its angle α with respect to a mediolateral direction X-X relative to the patient, and also the position of the implant <b>13</b> along the line of sectioning R_c1. The aforementioned adjustment means also make it possible to change the size of the patellar implant <b>13</b>, assuming the surgeon has available a set of such implants of different sizes held in the computer <b>2</b>.

As regards the femoral implant <b>12</b>, the aforementioned adjustment means advantageously allow modification of the size of this implant, its mediolateral position relative to the bone of the femur F, as is indicated by the double arrow <b>31</b> in <figref idrefs="DRAWINGS">FIG. 6</figref>, and/or its angular position relative to the femur about an overall vertical direction Z-Z fixed in advance, as is indicated by the double arrow <b>32</b>.

To facilitate the adjustment of the virtual implantations of the prosthetic components <b>12</b> and <b>13</b>, the representation in <figref idrefs="DRAWINGS">FIG. 6</figref> advantageously includes the outline, shown by broken lines <b>21</b>, in the plane P, of the prosthetic patellar button <b>131</b> in a predetermined position relative to the femoral implant <b>12</b>. This predetermined position, whose geometric characteristics are provided for example by the manufacturer of the prosthesis <b>10</b>, is the position considered to be optimal with a view to guaranteeing the best possible congruent configuration between the patellar and femoral components of the prosthesis. The surgeon is aided by this outline, since he is then able to modify the configurations of virtual implantation of the implants <b>12</b> and <b>13</b> so as to cause the patellar button <b>131</b> and the aforementioned outline <b>21</b> to come into coincidence with each other on the graph.

The congruence between the virtually implanted components <b>12</b> and <b>13</b> is advantageously checked when the patient's knee joint is in extension, as in <figref idrefs="DRAWINGS">FIGS. 4 and 5</figref>, but also when this joint is in flexion. Thus, for every flexed configuration of the knee selected by the surgeon, either on the basis of a corresponding computer command or by real bending of the patient's leg, whose movements are followed by the markers <b>5</b>, <b>6</b> and <b>7</b>, the computer displays a representation analogous to that in <figref idrefs="DRAWINGS">FIG. 6</figref>, including, if appropriate, an outline for helping the surgeon analogous to the outline <b>21</b>, and the data relating to the different outlines in the different configurations of flexion-extension of the knee are delivered to and stored in the computer <b>2</b>. The successive representations obtained are effected in the fixed plane P in relation to the patella R, while the section views of the femur F and of its virtual implant <b>12</b> develop as a function of the flexion-extension of the knee, according to the relative displacements between the patella and the femur. The adjustment of the virtual implantation of the prosthetic components <b>12</b> and <b>13</b> is thus possible irrespective of the degree of flexion of the patient's leg. The congruence can therefore be checked over the course of flexion most critical from the point of view of the risks of subsequent dislocations of the patella, that is to say for the degrees of flexion between about 0 and 30°.

Alternatively, the checking of the congruency and the adjustment of the virtual implantation of the prosthetic components <b>12</b> and <b>13</b> are carried out only for a discrete series of configurations of flexion of the knee joint, these configurations being chosen by the surgeon as being the most critical from the point of view of the risks of dislocation of the patella.

At the end of this third operating step, the surgeon has the computer <b>2</b> memorize the configurations of virtual implantation in the plane P of the prosthetic components <b>12</b> and <b>13</b> that he has set.

In a fourth step, the surgeon checks that the antero-posterior size of the femoral component <b>12</b> and patellar component <b>13</b> of the prosthesis is substantially identical to the corresponding anatomical size presented by the patient, so that the prosthesis, once implanted, does not significantly disturb the muscle tension and ligament tension of the patient's joint. To this end, the computer <b>2</b> provides the surgeon with a simultaneous and relative representation of the patella R provided virtually with the patellar implant <b>13</b> and of the lower part of the femur F provided virtually with the femoral implant <b>12</b>, specifically in a sagittal plane passing both through the patella and the prosthetic femoral trochlea <b>123</b>, as shown in <figref idrefs="DRAWINGS">FIG. 9</figref>.

To fully understand the benefit of this representation in the aforementioned sagittal plane, the section of the femoral implant <b>12</b> in this plane is shown in <figref idrefs="DRAWINGS">FIG. 7</figref>. In practice, the sagittal plane in question corresponds to the plane S indicated in <figref idrefs="DRAWINGS">FIG. 2</figref> passing through the base line of the trochlea <b>123</b>. <figref idrefs="DRAWINGS">FIG. 8</figref> shows, in the plane S, a line <b>22</b> which corresponds to the theoretical trajectory of the centre O of the patellar button <b>131</b> relative to the femoral implant <b>12</b> when this button is displaced in the prosthetic trochlea <b>123</b> during a movement of flexion-extension of the knee prosthesis <b>10</b>. The data relating to the line <b>22</b> are, for example, supplied by the manufacturer of the prosthesis <b>10</b>. This line is representative of an optimal implantation of the patellar component <b>13</b> from the point of view of the antero-posterior spacing between this component <b>13</b> and the femoral component <b>12</b>. In other words, to guarantee that the antero-posterior part of the contact between the patella button <b>131</b> of the subsequently fitted implant <b>13</b> and the femoral trochlea <b>123</b> of the subsequently fitted implant <b>12</b> is as close as possible to that associated with the prosthesis <b>10</b>, the implantations of the components <b>12</b> and <b>13</b> must as far as possible tend to place on the line <b>22</b> the centre O of the actually implanted patella button <b>131</b> in relation to the actually implanted femoral component <b>12</b>.

Thus, by virtue of the representation in <figref idrefs="DRAWINGS">FIG. 9</figref>, the surgeon is able to check, advantageously during a continuous stressing in flexion-extension of the leg, or for only some configurations of flexion of the knee joint chosen by the surgeon, that the centre of the virtual patella button <b>131</b> is situated on or in immediate proximity to the theoretical line <b>22</b> described above, of which the characteristic data have been delivered beforehand to the computer <b>2</b>. This is for example the case in <figref idrefs="DRAWINGS">FIG. 9</figref> where the knee joint is shown in approximately 60° flexion. The subsequent implantation of the components <b>12</b> and <b>13</b> will thus make it possible to obtain the expected prosthetic kinematics without significantly disturbing the surrounding muscles and ligaments of the knee that is being operated on.

In the event that the centre O is too far away from the line <b>22</b>, the surgeon modifies the implantation configuration of the virtual patellar implant <b>13</b> and/or the implantation configuration of the virtual femoral implant <b>12</b>. As regards the virtual patellar implantation, the surgeon, using adjustment means analogous to those described above in respect of <figref idrefs="DRAWINGS">FIG. 6</figref> and held by the computer <b>2</b> and the screen <b>8</b>, is able to modify the position of the section line R_c2of the patella in the sagittal plane S, in particular its distance e_sin relation to the summit of the anterior face of the patella and its angle β in relation to an antero-posterior direction Y-Y, and also the position of the implant <b>13</b> along the section line R_c2.

Alternatively, the outline of the line <b>22</b> is not supplied to the surgeon on the screen <b>8</b>, so that the surgeon adjusts the implantation parameters on the basis of only the display of the virtual implants <b>12</b> and <b>13</b> relative to the bones of the femur F and patella R.

In the event that a set of several patellar implants <b>13</b> of different sizes is available and that the data relating to this set have been supplied beforehand to the computer <b>2</b>, the aforementioned adjustment means also make it possible to change the size of the virtual implant <b>13</b> shown by the apparatus <b>1</b>.

As regards the femoral implant <b>12</b>, the adjustment means advantageously make it possible to modify the size of this implant if a set of several femoral implants is provided, its antero-posterior position relative to the femur F, as indicated by the double arrow <b>33</b> in <figref idrefs="DRAWINGS">FIG. 9</figref>, and/or its annular position relative to the femur about the mediolateral direction X-X as indicated by the double arrow <b>34</b>.

At the end of this fourth operating step, the surgeon has the computer <b>2</b> memorize the configurations of virtual implantation, in the plane S, of the prosthetic components <b>12</b> and <b>13</b> that he has set.

In a fifth operating step, the surgeon resects the upper end of the tibia T, the lower end of the femur F and the posterior part of the patella R. To do so, the surgeon uses ancillary cutting devices marked out permanently by the computer <b>2</b>, in such a way that the latter guides the surgeon's manoeuvres for resecting the aforementioned bones according to the implantation configurations that have previously been selected, in particular the patellar implantation configuration. The document US-A-2003/0212403 describes, for example, the use of tibial, femoral and patellar cutting guides that can be marked out by the computer <b>2</b>.

It will be noted that the patellar sectioning plane is deduced by the computer <b>2</b> from the data relating to the sectioning lines R_c1and R_c2which were set during the third and fourth operating steps.

In a sixth operating step, the surgeon implants the components <b>11</b>, <b>12</b> and <b>13</b> of the prosthesis <b>10</b> in the resected surfaces of the corresponding bones T, S and R.

The apparatus <b>1</b> according to the invention thus makes it possible to position the prosthesis <b>10</b> in an optimal manner for reducing as far as possible the risks of subsequent dislocations of the patellar component <b>13</b>.

It will be noted that the six operating steps described above are carried out during an actual surgical intervention, that is to say during which, for example, the patient is under anaesthetic.

Moreover, the different data recorded during the fitting of the prosthesis can be used to draw up a post-operative record and to thus characterize with precision the articulation capacities of the prosthesis in its implantation state. It will be noted, however, that the amount of data acquired is much less than that necessary for the functioning of a biomechanical simulator of the knee, and that the corresponding computing means of the apparatus according to the invention are less costly and less complex to operate.

Various refinements and variations of the implantation apparatus <b>1</b> described above are also conceivable. By way of example, the means for marking out the bones F, T and R are not limited to the infrared reflection markers; for example, markers sensitive to ultrasound or to electromagnetic fields can be used.